From the discovery of superconductivity in 1911 to the recent past, essentially all known superconducting materials were elemental metals (e.g., Hg, the first known superconductor) or metal alloys or intermetallic compounds (e.g., Nb.sub.3 Ge, probably the material with the highest transition temperature T.sub.c known prior to 1986).
Recently, superconductivity was discovered in a new class of materials, namely, metal oxides. See, for instance, J. G. Bednorz and K. A. Muller, Zeitschr. f. Physik B--Condensed Matter, Vol. 64, 189 (1986), which reports superconductivity in lanthanum barium copper oxide.
The above report stimulated worldwide research activity, which very quickly resulted in further significant progress. The progress has resulted, inter alia, to date in the discovery that compositions in the Y--Ba--Cu--O system can have superconductive transition temperatures T.sub.c above 77K, the boiling temperature of liquid N.sub.2 (see, for instance, M. K. Wu et al, Physical Review Letters, Vol. 58, Mar. 2, 1987, page 908; and P. H. Hor et al, ibid, page 911). Furthermore, it has resulted in the identification of the material phase that is responsible for the observed high temperature superconductivity, and in the discovery of composition and processing techniques that result in the formation of bulk samples of material that can be substantially single phase material and can have T.sub.c above 90K (see, for instance, R. J. Cava et al, Physical Review Letters, Vol. 58(16), pp. 1676-1679), incorporated herein by reference.
The excitement in the scientific and technical community that was created by the recent advances in superconductivity is at least in part due to the potentially immense technological impact of the availability of materials that are superconducting at temperatures that do not require refrigeration with expensive liquid He. Liquid nitrogen is generally considered to be one of the most advantageous cryogenic refrigerants, and attainment of superconductivity at or above liquid nitrogen temperature was a long-sought goal which until very recently appeared almost unreachable.
For a general overview of some potential applications of superconductors see, for instance, B. B. Schwartz and S. Foner, editors, Superconductor Applications: SQUIDS and MACHINES, and S. Foner and B. B. Schwartz, editors, Superconductor Material Science, Metallurgy, Fabrications, and Applications, Plenum Press 1981. Among the applications are power transmission lines, rotating machinery, and superconducting magnets for, e.g., fusion generators, MHD generators, particle accelerators, levitated vehicles, magnetic separation, and energy storage, as well as junction devices and detectors. It is expected that many of the above and other applications of superconductivity would materially benefit if high T.sub.c superconductive material could be used instead of the previously considered relatively low T.sub.c materials.
Two general approaches for forming superconductive oxide bodies are known to the art. Thin films are formed by deposition of material on a substrate (e.g., by sputtering, evaporation, or decomposition of a solution), followed by a heat treatment that produces the appropriate crystal structure and composition (typically by adjustment of the oxygen content). On the other hand, bulk bodies and thick films are generally produced by synthesizing a powder of the appropriate composition (e.g., YBa.sub.2 Cu.sub.3 O.sub.x, x.about.7), forming the powder into the desired shape (e.g., by hot pressing, drawing, extrusion, or silk screening of a slurry), and heat treating the resulting body such that sintering occurs, and such that the sintered material has the appropriate crystal structure and composition. A further method which comprises melting of the oxide powder and forming bulk bodies by solidification of the oxide melt is discussed below.
The critical temperature T.sub.c, i.e., the temperature at which a given body becomes superconductive, is an important parameter of a superconductor. Another important parameter is the maximum current density that can be supported by a body in the superconductive state. This "critical current density" J.sub.c decreases with both increasing temperature and increasing magnetic field.
Work to date has shown that at least some thin films of high T.sub.c superconductors (e.g., YBa.sub.2 Cu.sub.3 O.sub.7) can have high J.sub.c (of order 10.sup.6 A/cm.sup.2 at 77K), with J.sub.c being relatively weakly dependent on magnetic field. Work has also shown that, even though individual particles (crystallites) of superconductive oxides (e.g., YBa.sub.2 Cu.sub.3 O.sub.7) can have large internal critical current density J.sub.c (of order 10.sup.6 A/cm.sup.2), the critical current density of bulk bodies produced by sintering of the particles is relatively small, exemplarily of order 10.sup.3 A/cm.sup.2 in zero magnetic field (H=0), and strongly dependent on magnetic field. This huge difference between the J.sub.c of a single particle and of an assembly of particles is generally attributed to the presence of weak links between adjacent particles (by "weak links" we mean herein any inhomogeneity, frequently associated with the surface of a particle or with the contact between two particles, that limits the density of supercurrent that can flow). A critical current density of the order of 10.sup.3 A/cm.sup.2 at H=0 is generally thought to be too small for most technologically important applications. Furthermore, the J.sub.c of sintered bulk superconductive oxide bodies decreases rapidly as a function of magnetic field, further limiting the current that could be carried by such prior art bodies.
As discussed above, most bulk high T.sub.c superconductive bodies are produced by ceramic processing techniques that involve sintering of powder material at temperatures below the melting temperature of the material. See, for instance, D. W. Johnson et al, Advanced Ceramic Material, Vol. 2(3B), July 1987, pp. 364-371. However, recently work was reported that represents a significant departure from the conventional (i.e., ceramic) processing method since it involves melting of the metal oxide powder. See S. Jin et al, Applied Physics Letters, Vol. 51(12), pp. 943-945, (1987) and U.S. patent application Ser. No. 126,083, filed Nov. 27, 1987, which is a continuation-in-part of U.S. patent application Ser. No. 062,529, filed June 12, 1987, now abandoned. The "metallurgical" processing technique of Jin et al can result in essentially 100% dense, essentially single phase material in which the grains typically are of relatively large size and typically are non-randomly oriented. Bulk bodies produced by this technique can have substantially larger J.sub.c than has been reported for sintered bodies of the same composition, and, significantly, J.sub.c can decrease more slowly with increasing magnetic field than has been reported for sintered bodies. These improvements are thought to be due at least in part to improved intergranular contact and/or to the presence of orientational correlation between neighboring crystallites. However, even though the melting technique of Jin et al results in substantially improved J.sub.c, the observed behavior still suggests that J.sub.c is limited at least to some extent by weak links, possibly associated with compositional inhomogeneity.
In view of the immense economic potential of high T.sub.c superconductors, a simple, scalable processing method which is readily applicable to continuous processing and the formation of composite structures, and which has the potential for producing improved bodies, especially material with improved compositional uniformity, would be of great interest. This application discloses such a method. Furthermore, the disclosed method is believed to have broader applicability. For instance, it is thought that it can be advantageously used to produce at least some non-superconductive ceramics, and to produce such nonoxidic materials as aluminum nitride.